The present invention is directed to polyester tray packages having lidding films which are directly sealable to the polyester tray and which contain glycol-modified copolyester sealant layers. More specifically, the present invention is directed to polyethylene terephthalate (PET) tray packages having lidding films which include glycol-modified polyethyelene terephthalate sealant layers.
Polyethylene terephthalate (PET) trays are employed in packaging a wide variety of products. PET trays are especially preferred for use in packaging various food products because of the polymers superior mechanical properties, transparency, low extractability by contents and good flavor retention of contents. PET trays may be foamed or non-foamed, depending on the required end-use application, and are preferred over foamed and non-foamed polystyrene trays because of their recyclability.
However, PET has a very high melting point of 260xc2x0 C. which makes heat sealing difficult. That is, readily available olefin-based lidding films cannot be heat sealed to PET trays because such films typically have a sealing window with an upper limit well below 260xc2x0 C. and will burn at sealing temperatures above the upper end of that range. Some PET films can be sealed to PET trays at about 260xc2x0 C. but such an elevated heat sealing operation is not commercially practicable in a typical packaging facility.
Monolayer films which consist of glycol-modified polyethylene terephthalate have been found to provide adequate seals to foamed and nonfoamed PET trays. Such glycol-modified PET films can be sealed to PET trays in a sealing window range of from about 115xc2x0 C. to about 210xc2x0 C. However, such monolayer films do not provide gas barrier properties which are desirable for certain end-use applications.
In a first aspect, the present invention is directed to a package which includes a polyester support member, a product contained on the support member, and a multilayer lidding film containing the product and sealed to the periphery of the support member, the lidding film including a sealant layer of a polyester copolymer having greater than about 50% by mole of glycol polymerization units.
In a second aspect, the present invention is directed to a package which includes a polyester support member, a product contained on the support member, and a lidding film containing the product and sealed to the periphery of the support member, the lidding film including a sealant layer of a polyester copolymer having greater than about 50% by mole of glycol polymerization units and at least one further polymeric component.
In a third aspect, the present invention is directed to a method for making a package which includes the steps of coextruding a multilayer lidding film which includes a sealant layer of a polyester copolymer having greater than about 50% by mole of glycol polymerization units, providing a polyester support member, placing a product on the support member, extending the lidding film above the support member and product with the sealant layer being immediately above and adjacent to the support member and the product, and sealing the lidding film to the support member such that the product is enclosed by the film and such that the sealant layer is directly sealed to periphery of the support member.
In a fourth aspect, the present invention is directed to a method for making a package which includes the steps of extruding a multilayer lidding film which includes a sealant layer of a blend of a polyester copolymer having greater than about 50% by mole of glycol polymerization units and at least one further polymeric component, providing a polyester support member, placing a product on the support member, extending the lidding film above the support member and product with the sealant layer being immediately above and adjacent to the support member and the product, and sealing the lidding film to the support member such that the product is enclosed by the film and such that the sealant layer is directly sealed to periphery of the support member.
As used herein, the phrase xe2x80x9cabuse layerxe2x80x9d refers to an outer film layer and/or an inner film layer, so long as the film layer serves to resist abrasion, puncture, and other potential causes of reduction of package integrity, as well as potential causes of reduction of package appearance quality. Abuse layers can comprise any polymer, so long as the polymer contributes to achieving an integrity goal and/or an appearance goal.
As used herein, xe2x80x9coxygen transmission ratexe2x80x9d, also referred to as xe2x80x9cOTRxe2x80x9d and xe2x80x9coxygen permeabilityxe2x80x9d, is measured according to ASTM D 3985, a test known to those of skill in the film art.
As used herein, the term xe2x80x9claminationxe2x80x9d, and the phrase xe2x80x9claminated filmxe2x80x9d, refer to the process, and resulting product, made by bonding together two or more layers of film or other materials. Lamination can be accomplished by joining layers with adhesives, joining with heat and pressure, and even spread coating and extrusion coating. The term laminate is also inclusive of coextruded multilayer films comprising one or more tie layers.
As used herein, the term xe2x80x9corientedxe2x80x9d refers to a polymer-containing material which has been stretched at the softening temperature but below the melting temperature, followed by being xe2x80x9csetxe2x80x9d in the stretched configuration by cooling the material while substantially retaining the stretched dimensions. Upon subsequently heating unrestrained, unannealed, oriented polymer-containing material to its orientation temperature, heat shrinkage is produced almost to the original unstretched, i.e., pre-oriented dimensions.
As used herein, the phrases hot blown or hot blowing refer an extrusion process as well as a characteristic of the resultant film. In a hot blown process, molten plastic material is forced through an annular die and is immediately inflated to a diameter typically two to four times that of the melt exiting the die. Hot blown films do possess orientation; but since the films are oriented at or very near to the melt temperature they will only shrink when heated to that high temperature which is much higher than the temperature required to shrink conventional oriented films. For this reason, hot blown films do not exhibit shrink within the temperature range useful for food packaging since the film must be heated to nearly melting in order to observe any appreciable degree of shrinkage. Hot blown films may be referred to as melt state oriented films.
As used herein, the term xe2x80x9ccomonomerxe2x80x9d refers to a monomer which is copolymerized with at least one different monomer in a copolymerization reaction, the result of which is a copolymer.
As used herein, the term xe2x80x9cpolymerxe2x80x9d refers to the product of a polymerization reaction, and is inclusive of homopolymers, copolymers, terpolymers, etc. In general, the layers of a film can consist essentially of a single polymer, or can have still additional polymers blended therewith.
As used herein, the term xe2x80x9chomopolymerxe2x80x9d is used with reference to a polymer resulting from the polymerization of a single monomer, i.e., a polymer consisting essentially of a single type of repeating unit.
As used herein, the term xe2x80x9ccopolymerxe2x80x9d refers to polymers formed by the polymerization reaction of at least two different monomers. For example, the term xe2x80x9ccopolymerxe2x80x9d includes the copolymerization reaction product of ethylene and an xcex1-olefin, such as 1-hexene. However, the term xe2x80x9ccopolymerxe2x80x9d is also inclusive of, for example, the copolymerization of a mixture of ethylene, propylene, 1-hexene, and 1-octene.
As used herein, the term xe2x80x9cpolymerizationxe2x80x9d is inclusive of homopolymerizations, copolymerizations, terpolymerizations, etc., and includes all types of copolymerizations such as random, graft, block, etc. In general, the polymers, in the films used in accordance with the present invention, can be prepared in accordance with any suitable catalytic polymerization process, including solution polymerization, slurry polymerization, gas phase polymerization, and high pressure polymerization processes.
Slurry polymerization processes generally use superatmospheric pressures and temperatures in the range of 40xc2x0-100xc2x0 C. In a slurry polymerization, a suspension of solid, particulate polymer is formed in a liquid polymerization medium to which ethylene and comonomers and often hydrogen along with catalyst are added. The liquid employed in the polymerization medium can be an alkane, cycloalkane, or an aromatic hydrocarbon such as toluene, ethylbenzene or xylene. The medium employed should be liquid under the conditions of polymerization, and relatively inert. Preferably, hexane or toluene is employed.
Alternatively, gas-phase polymerization process utilizes superatmospheric pressure and temperature in the range of about 50xc2x0-120xc2x0 C. Gas phase polymerization can be performed in a stirred or fluidized bed of catalyst and product particles in a pressure vessel adapted to permit the separation of product particles from unreacted gases. Ethylene, comonomer, hydrogen and an inert diluent gas such as nitrogen can be introduced or recirculated so as to maintain the particles at temperatures of 50xc2x0-120xc2x0 C. Triethylaluminum may be added as needed as a scavenger of water, oxygen, and other impurities. Polymer product can be withdrawn continuously or semicontinuously, at a rate such as to maintain a constant product inventory in the reactor. After polymerization and deactivation of the catalyst, the product polymer can be recovered by any suitable means. In commercial practice, the polymer product can be recovered directly from the gas phase reactor, freed of residual monomer with a nitrogen purge, and used without further deactivation or catalyst removal.
High pressure polymerization processes utilize a catalyst system comprising a cyclopentadienyl-transition metal compound and an alumoxane compound. It is important, in the high-pressure process, that the polymerization temperature be above about 120xc2x0 C., but below the decomposition temperature of the polymer product. It is also important that the polymerization pressure be above about 500 bar (kg/cm2). In those situations wherein the molecular weight of the polymer product that would be produced at a given set of operating conditions is higher than desired, any of the techniques known in the art for control of molecular weight, such as the use of hydrogen or reactor temperature, may be used in the process of this invention.
As used herein, the term xe2x80x9ccopolymerizationxe2x80x9d refers to the simultaneous polymerization of two or more monomers.
As used herein, a copolymer identified in terms of a plurality of monomers, e.g., xe2x80x9cpropylene/ethylene copolymerxe2x80x9d, refers to a copolymer in which either monomer copolymerizes in a higher weight or molar percent. However, the first listed monomer preferably is polymerized in a higher weight percent than the second listed monomer, and, for copolymers which are terpolymers, quadripolymers, etc., preferably, the first monomer copolymerizes in a higher weight percent than the second monomer, and the second monomer copolymerizes in a higher weight percent than the third monomer, etc.
As used herein, terminology employing a xe2x80x9c/xe2x80x9d with respect to the chemical identity of a copolymer (e.g., xe2x80x9can ethylene/xcex1-olefin copolymerxe2x80x9d), identifies the comonomers which are copolymerized to produce the copolymer. Such phrases as xe2x80x9cethylene xcex1-olefin copolymerxe2x80x9d is the respective equivalent of xe2x80x9cethylene/xcex1-olefin copolymer.xe2x80x9d
As used herein, the phrase xe2x80x9cheterogeneous polymerxe2x80x9d refers to polymerization reaction products of relatively wide variation in molecular weight and relatively wide variation in composition distribution, i.e., polymers made, for example, using conventional Ziegler-Natta catalysts. Heterogeneous polymers are useful in various layers of the film used in the present invention. Such polymers typically contain a relatively wide variety of chain lengths and comonomer percentages.
As used herein, the phrase xe2x80x9cheterogeneous catalystxe2x80x9d refers to a catalyst suitable for use in the polymerization of heterogeneous polymers, as defined above. Heterogeneous catalysts are comprised of several kinds of active sites which differ in Lewis acidity and steric environment. Ziegler-Natta catalysts are heterogeneous catalysts. Examples of Ziegler-Natta heterogeneous systems include metal halides activated by an organometallic co-catalyst, such as titanium chloride, optionally containing magnesium chloride, complexed to trialkyl aluminum and may be found in patents such as U.S. Pat. No. 4,302,565, to GOEKE, et. al., and U.S. Pat. No. 4,302,566, to KAROL, et. al., both of which are hereby incorporated, in their entireties, by reference thereto.
As used herein, the phrase xe2x80x9chomogeneous polymerxe2x80x9d refers to polymerization reaction products of relatively narrow molecular weight distribution and relatively narrow composition distribution. Homogeneous polymers are useful in various layers of the multilayer film used in the present invention. Homogeneous polymers exhibit a relatively even sequencing of comonomers within a chain, the mirroring of sequence distribution in all chains, and the similarity of length of all chains, and are typically prepared using metallocene, or other single-site type catalysis.
More particularly, homogeneous copolymers may be characterized by one or more methods known to those of skill in the art, such as molecular weight distribution (Mw/Mn), composition distribution breadth index (CDBI), and narrow melting point range and single melt point behavior. The molecular weight distribution (Mw/Mn), also known as polydispersity, may be determined by gel permeation chromatography. The homogeneous copolymers useful in this invention will have a (Mw/Mn) of less than 2.7. Preferably, the (Mw/Mn) will have a range of about 1.9 to 2.5. More preferably, the (Mw/Mn) will have a range of about 1.9 to 2.3. The composition distribution breadth index (CDBI) of such homogeneous copolymers will generally be greater than about 70 percent. The CDBI is defined as the weight percent of the copolymer molecules having a comonomer content within 50 percent (i.e., plus or minus 50%) of the median total molar comonomer content. The CDBI of linear polyethylene, which does not contain a comonomer, is defined to be 100%. The Composition Distribution Breadth Index (CDBI) is determined via the technique of Temperature Rising Elution Fractionation (TREF). CDBI determination clearly distinguishes the homogeneous copolymers used in the present invention (narrow composition distribution as assessed by CDBI values generally above 70%) from VLDPEs available commercially which generally have a broad composition distribution as assessed by CDBI values generally less than 55%. The CDBI of a copolymer is readily calculated from data obtained from techniques known in the art, such as, for example, temperature rising elution fractionation as described, for example, in Wild et. al., J. Poly. Sci. Poly. Phys. Ed., Vol. 20, p.441 (1982). Preferably, the homogeneous copolymers have a CDBI greater than about 70%, i.e., a CDBI of from about 70% to 99%. In general, the homogeneous copolymers in the lidding films of the present invention also exhibit a relatively narrow melting point range, in comparison with xe2x80x9cheterogeneous copolymersxe2x80x9d, i.e., polymers having a CDBI of less than 55%. Preferably, the homogeneous copolymers exhibit an essentially singular melting point characteristic, with a peak melting point (Tm), as determined by Differential Scanning Colorimetry (DSC), of from about 60xc2x0 C. to 110xc2x0 C. Preferably the homogeneous copolymer has a DSC peak Tm of from about 80xc2x0 C. to 100xc2x0 C. As used herein, the phrase xe2x80x9cessentially single melting pointxe2x80x9d means that at least about 80%, by weight, of the material corresponds to a single Tm peak at a temperature within the range of from about 60xc2x0 C. to 110xc2x0 C., and essentially no substantial fraction of the material has a peak melting point in excess of about 115xc2x0 C., as determined by DSC analysis. DSC measurements are made on a Perkin Elmer System 7 Thermal Analysis System. Melting information reported are second melting data, i.e., the sample is heated at a programmed rate of 10xc2x0 C./min. to a temperature below its critical range. The sample is then reheated (2nd melting) at a programmed rate of 10xc2x0 C./min. The presence of higher melting peaks is detrimental to film properties such as haze, and compromises the chances for meaningful reduction in the seal initiation temperature of the final film.
A homogeneous ethylene/xcex1-olefin copolymer can, in general, be prepared by the copolymerization of ethylene and any one or more xcex1-olefin. Preferably, the xcex1-olefin is a C3-C20 xcex1-monoolefin, more preferably, a C4-C12 xcex1-monoolefin, still more preferably, a C4-C8 xcex1-monoolefin. Still more preferably, the xcex1-olefin comprises at least one member selected from the group consisting of butene-1, hexene-1, and octene-1, i.e., 1-butene, 1-hexene, and 1-octene, respectively. Most preferably, the xcex1-olefin comprises octene-1, and/or a blend of hexene-1 and butene-1.
Processes for preparing homogeneous polymers are disclosed in U.S. Pat. No. 5,206,075, U.S. Pat. No. 5,241,031, and PCT International Application WO 93/03093, each of which is hereby incorporated by reference thereto, in its entirety. Further details regarding the production and use of one genus of homogeneous ethylene/xcex1-olefin copolymers are disclosed in U.S. Pat. No. 5,206,075, to HODGSON, Jr.; U.S. Pat. No. 5,241,031, to MEHTA; PCT International Publication Number WO 93/03093, in the name of Exxon Chemical Company; PCT International Publication Number WO 90/03414, in the name of Exxon Chemical Patents, Inc., all four of which are hereby incorporated in their entireties, by reference there. Still another genus of homogeneous ethylene/xcex1-olefin copolymers is disclosed in U.S. Pat. No. 5,272,236, to LAI, et. al., and U.S. Pat. No. 5,278,272, to LAI, et. al., both of which are hereby incorporated in their entireties, by reference thereto.
As used herein, the phrase xe2x80x9chomogeneous catalystxe2x80x9d refers to a catalyst suitable for use in the polymerization of homogeneous polymers, as defined above. Homogeneous catalysts are also referred to as xe2x80x9csingle site catalystsxe2x80x9d, due to the fact that such catalysts typically have only one type of catalytic site, which is believed to be the basis for the homogeneity of the polymers they catalyze the polymerization of.
As used herein, the phrase xe2x80x9cvinyl aromaticxe2x80x9d, with respect to monomers, refers to styrene, vinyl toluene, vinylnaphthalene, and vinylanthracene, with or without one or more substituents (for hydrogens) present on the aromatic ring(s), and/or the olefin carbon connected to the aromatic ring. Furthermore, this phrase is used herein with reference to polymerization units of the above monomers. Preferably, the vinyl aromatic monomer is styrene.
As used herein, the phrase xe2x80x9calpha-olefinxe2x80x9d, and the phrase xe2x80x9calpha-olefin monomerxe2x80x9d, refer to olefinic compounds, whether unsubstituted or substituted, in which the first two carbon atoms in the chain have a double bond there between. Furthermore, as used herein, both of these phrases are inclusive of ethylene and propylene.
As used herein, the phrase xe2x80x9cpolymerization unitxe2x80x9d refers to a unit of a polymer derived from a monomer used in the polymerization reaction. For example, the phrase xe2x80x9calpha-olefin polymerization unitsxe2x80x9d refers to a unit in, for example, an alpha-olefin/vinyl aromatic copolymer, the polymerization unit being that residue which is derived from the alpha-olefin monomer after it reacts to become a component of the polymer chain.
As used herein, the phrase xe2x80x9cvinyl aromatic polymerization unitxe2x80x9d refers to a corresponding polymerization unit of the polymer from the polymerization, which is the residue derived from the vinyl aromatic monomer after it reacts to become a component of the polymer chain.
As used herein, copolymers, terpolymers, etc. are named in terms of the monomers from which they are produced. For example, an xe2x80x9cethylene/alpha-olefin copolymerxe2x80x9d is a copolymer comprising polymerization units derived from the copolymerization of ethylene monomer and alpha-olefin monomer, with or without additional comonomer(s). Likewise, an alpha-olefin/vinyl aromatic copolymer is a copolymer comprising polymerization units derived from the copolymerization of alpha-olefin monomer with vinyl aromatic comonomer, with or without additional comonomer(s).
As used herein, the term xe2x80x9cpolyolefinxe2x80x9d refers to any polymerized olefin, which can be linear, branched, cyclic, aliphatic, aromatic, substituted, or unsubstituted. More specifically, included in the term polyolefin are homopolymers of olefins, copolymers of olefins, copolymers of an olefin and an non-olefinic comonomer copolymerizable with the olefin, such as vinyl monomers, modified polymers thereof, and the like. Specific examples include polypropylene homopolymers, polyethylene homopolymers, poly-butene, propylene/xcex1-olefin copolymers, ethylene/xcex1-olefin copolymers, butene/xcex1-olefin copolymers, ethylene/vinyl acetate copolymers, ethylene/ethyl acrylate copolymers, ethylene/butyl acrylate copolymers, ethylene/methyl acrylate copolymers, ethylene/acrylic acid copolymers, ethylene/methacrylic acid copolymers, modified polyolefin resins, ionomer resins, polymethylpentene, etc. The modified polyolefin resins include modified polymers prepared by copolymerizing the homopolymer of the olefin or copolymer thereof with an unsaturated carboxylic acid, e.g., maleic acid, fumaric acid or the like, or a derivative thereof such as the anhydride, ester or metal salt or the like. It could also be obtained by incorporating into the olefin homopolymer or copolymer, an unsaturated carboxylic acid, e.g., maleic acid, fumaric acid or the like, or a derivative thereof such as the anhydride, ester or metal salt or the like.
As used herein, terms identifying polymers, such as xe2x80x9cpolyamidexe2x80x9d, xe2x80x9cpolyesterxe2x80x9d, polyethylenexe2x80x9d, etc. are inclusive of not only polymers comprising repeating units derived from monomers known to polymerize to form a polymer of the named type, but are also inclusive of comonomers, derivatives, etc. which can copolymerize with monomers known to polymerize to produce the named polymer. For example, the term xe2x80x9cpolyamidexe2x80x9d encompasses both polymers comprising repeating units derived from monomers, such as caprolactam, which polymerize to form a polyamide, as well as copolymers derived from the copolymerization of caprolactam with a comonomer which when polymerized alone does not result in the formation of a polyamide. Furthermore, terms identifying polymers are also inclusive of xe2x80x9cblendsxe2x80x9d of such polymers with other polymers of a different type.
As used herein, the term xe2x80x9cpolypropylenexe2x80x9d refers to any polymer comprising propylene polymerization units, regardless of whether the polymer is a homopolymer or a copolymer, and further includes blends of such homopolymers and copolymers. The phrase xe2x80x9cpropylene polymerization unitsxe2x80x9d, as used herein, refers to polymerization units in a polymer chain, the repeating units being derived from the polymerization of unsubstituted propylene monomer and/or substituted propylene polymer, the double bond being opened in the polymerization reaction.
As used herein, the phrase xe2x80x9canhydride functionalityxe2x80x9d refers to any form of anhydride functionality, such as the anhydride of maleic acid, fumaric acid, etc., whether blended with one or more polymers, grafted onto a polymer, or copolymerized with a polymer, and, in general, is also inclusive of derivatives of such functionalities, such as acids, esters, and metal salts derived therefrom.
As used herein, the phrase xe2x80x9cmodified polymerxe2x80x9d, as well as more specific phrases such as xe2x80x9cmodified ethylene vinyl acetate copolymerxe2x80x9d, and xe2x80x9cmodified polyolefinxe2x80x9d refer to such polymers having an anhydride functionality, as defined immediately above, grafted thereon and/or copolymerized therewith and/or blended therewith. Preferably, such modified polymers have the anhydride functionality grafted on or polymerized therewith, as opposed to merely blended therewith.
As used herein, the phrase xe2x80x9canhydride-containing polymerxe2x80x9d refers to one or more of the following: (1) polymers obtained by copolymerizing an anhydride-containing monomer with a second, different monomer, and (2) anhydride grafted copolymers, and (3) a mixture of a polymer and an anhydride-containing compound.
As used herein, the phrase xe2x80x9cethylene/alpha-olefin copolymerxe2x80x9d, and xe2x80x9cethylene/xcex1-olefin copolymerxe2x80x9d, refer to such heterogeneous materials as linear low density polyethylene (LLDPE), and very low and ultra low density polyethylene (VLDPE and ULDPE); and homogeneous polymers such as metallocene catalyzed polymers such as EXACT (TM) materials supplied by Exxon, and TAFMER (TM) materials supplied by Mitsui Petrochemical Corporation. These materials generally include copolymers of ethylene with one or more comonomers selected from C4 to C10 alpha-olefins such as butene-1 (i.e., 1-butene), hexene-1, octene-1, etc. in which the molecules of the copolymers comprise long chains with relatively few side chain branches or cross-linked structures. This molecular structure is to be contrasted with conventional low or medium density polyethylenes which are more highly branched than their respective counterparts. LLDPE, as used herein, has a density usually in the range of from about 0.91 grams per cubic centimeter to about 0.94 grams per cubic centimeter. Other ethylene/xcex1-olefin copolymers, such as the long chain branched homogeneous ethylene/xcex1-olefin copolymers available from the Dow Chemical Company, known as AFFINITY (TM) resins, are also included as another type of alpha-olefin copolymer useful in the present invention.
In general, the ethylene/xcex1-olefin copolymer comprises a copolymer resulting from the copolymerization of from about 80 to 99 weight percent ethylene and from 1 to 20 weight percent xcex1-olefin. Preferably, the ethylene xcex1-olefin copolymer comprises a copolymer resulting from the copolymerization of from about 85 to 95 weight percent ethylene and from 5 to 15 weight percent xcex1-olefin.
As used herein, the phrases xe2x80x9cinner layer,xe2x80x9d xe2x80x9cinternal layerxe2x80x9d and interior layer refer to any layer, of a multilayer film, having both of its principal surfaces directly adhered to another layer of the film.
As used herein, the phrases xe2x80x9couter layer,xe2x80x9d external layer and exterior layer refer to any film layer of a multilayer film having only one of its principal surfaces directly adhered to another layer of the film.
As used herein, the phrase xe2x80x9cinside layerxe2x80x9d refers to the outer layer, of a multilayer film packaging a product, which is closest to the product, relative to the other layers of the multilayer film.
As used herein, the phrase xe2x80x9coutside layerxe2x80x9d refers to the outer layer, of a multilayer film packaging a product, which is furthest from the product relative to the other layers of the multilayer film.
As used herein, the phrase xe2x80x9cdirectly adheredxe2x80x9d, as applied to film layers, is defined as adhesion of the subject film layer to the object film layer, without a tie layer, adhesive, or other layer there between. In contrast, as used herein, the word xe2x80x9cbetweenxe2x80x9d, as applied to a film layer expressed as being between two other specified layers, includes both direct adherence of the subject layer between to the two other layers it is between, as well as including a lack of direct adherence to either or both of the two other layers the subject layer is between, i.e., one or more additional layers can be imposed between the subject layer and one or more of the layers the subject layer is between.
As used herein, the term xe2x80x9ccorexe2x80x9d, and the phrase xe2x80x9ccore layerxe2x80x9d, as applied to multilayer films, refer to any inner (i.e., internal) film layer which has a primary function other than serving as an adhesive or compatibilizer for adhering two layers to one another. Usually, the core layer or layers provide the multilayer film with a desired level of strength, i.e., modulus, and/or optics, and/or added abuse resistance, and/or specific impermeability.
As used herein, the term xe2x80x9csealedxe2x80x9d refers to any and all means of closing the package, such as heat sealing via hot air and/or heated bar, and ultrasonic sealing.
As used herein, the phrases xe2x80x9cseal layerxe2x80x9d, xe2x80x9csealing layerxe2x80x9d, xe2x80x9cheat sealing layerxe2x80x9d and xe2x80x9csealant layerxe2x80x9d, refers to an outer film layer involved in the sealing of the lidding film to the polystyrene support member.
As used herein, the phrase xe2x80x9ctie layerxe2x80x9d refers to any inner layer having the primary purpose of adhering two layers to one another. In general, suitable polymers for use in tie layer include polymers having polar functional groups.
As used herein, the phrase xe2x80x9cskin layerxe2x80x9d refers to an outer layer of a multilayer film used in a package containing a product, wherein the film is used to make the package so that the outer layer is an outside layer with respect to the package. Such outside outer film layers are subject to abuse during storage and handling of the packaged product.
As used herein, the phrase xe2x80x9cbulk layerxe2x80x9d refers to any layer of a film which is present for the purpose of increasing the abuse-resistance, toughness, modulus, etc., of a multilayer film. Bulk layers generally comprise polymers which are inexpensive relative to other polymers in the film which provide some specific purpose unrelated to abuse-resistance, modulus, etc.
As used herein, the term xe2x80x9cextrusionxe2x80x9d is used with reference to the process of forming continuous shapes by forcing a molten plastic material through a die, followed by cooling or chemical hardening. Immediately prior to extrusion through the die, the relatively high-viscosity polymeric material is fed into a rotating screw of variable pitch, which forces it through the die.
As used herein, the term xe2x80x9ccoextrusionxe2x80x9d refers to the process of extruding two or more materials through a single die with two or more orifices arranged so that the extrudates merge and weld together into a laminar structure before chilling, i.e., quenching. Coextrusion can be employed in film blowing, free film extrusion, and extrusion coating processes.
As used herein, the phrase xe2x80x9cmachine directionxe2x80x9d, herein abbreviated xe2x80x9cMDxe2x80x9d, refers to a direction xe2x80x9calong the lengthxe2x80x9d of the film, i.e., in the direction of the film as the film is formed during extrusion and/or coating.
As used herein, the phrase xe2x80x9ctransverse directionxe2x80x9d, herein abbreviated xe2x80x9cTDxe2x80x9d, refers to a direction across the film, perpendicular to the machine or longitudinal direction.
As used herein, the phrase xe2x80x9cfree shrinkxe2x80x9d refers to the percent dimensional change in a 10 cmxc3x9710 cm specimen of film, when subjected to selected heat, as measured by ASTM D 2732, as known to those of skill in the art.